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Old 26th February 2009, 01:48 PM   #1
raghs is offline raghs  India
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Question Car Amp LA4440

Hi All,

I am trying to build an amplifier around LA4440. There is also a preamp, which is feeding the input to the main amp. I am driving my preamp from the RCA output of a car tape.

When the input to the preamp is not connected to the car tape, if I increase the volume of the amplifier I hear loud humming.

If I connect the car tape RCA output to the preamp input and increase the volume of main amp, then there is a loud flutter.

I have been breaking my head on this!
I checked the pot meters and they are properly grounded..
Can anyone tell me what do I need to solve the problems mentioned above??
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Old 26th February 2009, 01:53 PM   #2
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Ground the LA4440 amp and preamp to the chassis of the head unit. Do not let it ground through any other point. Does that reduce the noise?
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Old 26th February 2009, 03:56 PM   #3
raghs is offline raghs  India
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Thanks Perry Babin, Will try & let you know
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Old 2nd March 2009, 02:04 PM   #4
raghs is offline raghs  India
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Hi,

I tried what you told, but still there is humming. Also the preamp impedances are not matching, so how can I match them?
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Old 2nd March 2009, 02:15 PM   #5
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Was the humming reduced?

Is the hum related to the engine speed?

Can you post a schematic of the circuit you're using?
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Old 4th March 2009, 03:14 PM   #6
raghs is offline raghs  India
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Hi Perry,

I am trying out the circuit using transformers right now and the humming has gone away completely. thanks to your trick - I grounded the amp & preamp to the head unit.

I have a new question for you:
The pre-amp output from my tape is basically a pseudo signal. What they have done is they have put a 22k resistor in series with 1MF electrolytic capacitor to the speaker out and provided a new RCA jack!!

Now when I am feeding this output to my preamp input, the sound is muffled..its not clear. I guess the impedances are not matching? What do I need to do for this??
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File Type: gif schematics.gif (2.7 KB, 334 views)
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Old 4th March 2009, 03:21 PM   #7
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I don't think the problem is an impedance mismatch unless the input of the amplifier has a relatively large RF shunt capacitor. Can you provide a more detailed schematic?
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Old 4th March 2009, 03:28 PM   #8
raghs is offline raghs  India
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Which schematic do you need? The preamp of the circuit which feeds the output to main amp?

Basically I am giving the output of the headunit preamp to the preamp of my circuit, which in turn feeds the output to the main amp..does that make sense??
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Old 4th March 2009, 03:31 PM   #9
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Without knowing the circuit for the la4440 amp, I can't determine why the signal sounds muffled.
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Old 5th March 2009, 08:43 AM   #10
raghs is offline raghs  India
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Hi Perry - I am using the LA4440 in bridged mode, here's the schematic for the same. I got this circuit readymade from the market. The schematic just shows the LA4440 in bridged mode, I dont have the circuit diagram for the preamp & bass-treble module
Basicall the pre-amp is using a chip - HWCT35244 and 2 transistors BC548. I could not find any information or datasheet on HWCT35244

Eagerly awaiting inputs from you
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File Type: jpg la4440.jpg (16.5 KB, 324 views)
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